1. Field of the Invention
The present invention relates to a plasma processing or treatment apparatus for an object such as a semiconductor wafer.
2. Related Art
A plasma processing or treatment apparatus is configured such that a plasma is generated by vacuum-charging the interior of a processing vessel in which exists a processing gas, and that plasma is used to implement a predetermined treatment on an object such as a semiconductor wafer. In the prior art, such plasma treatment apparatuses are widely used throughout the semiconductor fabrication process, in steps such as in sputtering, ashing, CVD, and etching.
In a typical plasma treatment apparatus, a pair of horizontal, flat electrodes that face each other act as the plasma generation source. In this type of apparatus, the relationship of the electrode configuration means that a comparatively high gas pressure of, for example, several hundreds of mmTorr is used, and thus problems can occur such as ions within the plasma impacting against the electrodes and causing sputtering of the electrodes, which will lead to impurities being generated from the electrodes. For that reason, this type of apparatus is difficult to adapt to cope with recent ultra-fine processing techniques at a half-micron and greater. That is why various plasma treatment apparatuses are currently being developed to generate high-density plasmas under hard vacuum conditions. Examples of these plasma treatment apparatuses include a magnetron style that uses orthogonal electromagnetic fields, a radio-frequency inductively coupled style that uses radio-frequency electromagnetic energy, and, most recently, an apparatus that uses helicon, or helical waves generated from electromagnetic waves propagated parallel to a magnetic field.
A radio-frequency inductively coupled type of plasma treatment apparatus is provided with: a processing vessel that has a gas supply portion for supplying a processing gas and a gas exhaust portion for exhausting the gas after the treatment, and which maintains a predetermined degree of vacuum therein; a plasma generation means arranged on an upper surface of the processing vessel; and a holder device that holds a semiconductor wafer on which a predetermined treatment is to be performed in a plasma produced by the plasma generation means. The processing vessel is provided with a main body formed from an electrically conductive material such as stainless steel, with an upper portion formed from an insulating material that is transparent to radio-frequency waves, such as quartz, mounted on an upper end of the main body. Each of the above mentioned gas supply portion and gas exhaust portion is attached to the peripheral surface of the main body. The plasma generation means is provided with, for example, a one-turn antenna and a radio-frequency power source connected to this antenna by a matching circuit, with the configuration being such that radio-frequency electric power from the radio-frequency power source is supplied to the antenna through the matching circuit, radio-frequency waves are generated within the processing vessel from the antenna, and the process gas is activated by this electromagnetic energy to cause the generation of a plasma. The holder device is configured such that another radio-frequency power source is connected thereto with a blocking capacitor therebetween, radio-frequency electric power is supplied from this radio-frequency power source, and a self-bias potential is maintained with respect to the plasma's potential, via the blocking capacitor. When the above mentioned plasma treatment apparatus is used to physically etch a semiconductor wafer using argon, the pressure of the argon within the processing vessel is first adjusted to 10 mmTorr, with the semiconductor wafer held on the holder device. Next, when predetermined electric power from the radio-frequency power source is supplied to the antenna via the matching circuit, radio-frequency waves are generated within the processing vessel from the antenna, this electromagnetic energy excites the argon to form a plasma, and thus a high-density plasma of, for example, the order of 10.sup.11 cm.sup.-3 is generated. At the same time, since 100-kHz radio-frequency electric power is supplied through the blocking capacitor from the radio-frequency power source in the holder device, an ion sheath is formed between the holder device and the plasma, and argon ions from the plasma impact against the semiconductor wafer to perform the predetermined etching.
The previously mentioned prior art helical wave plasma treatment apparatus is provided with: a processing vessel having a gas supply means for supplying a process gas and a gas exhaust means for exhausting gases after the treatment, and is maintained at a predetermined degree of vacuum; a helical wave plasma generation means that surrounds an application portion forming part of the processing vessel; and a holder device for holding a semiconductor wafer that is to be subjected to a predetermined treatment by the helical wave plasma generated from the process gas by the action of the helical wave plasma generation means. This application portion is formed of an insulating material that is transparent to electromagnetic waves, such as quartz, and the above described main body connected to a lower end aperture of the application portion is formed of an electrically conductive material such as stainless steel. The helical wave plasma generation means is also provided with an antenna surrounding the outer peripheral surface of the application portion and an electromagnetic coil also surrounding the application portion, but further out from the antenna. The configuration is such that electromagnetic waves from the antenna propagate parallel to the magnetic field shaped by the electromagnetic coil; the electromagnetic waves are subjected to the action of the magnetic field as they propagate through the plasma, and thus generate helical waves; and a high-density helicon wave plasma is generated by these helical waves.
When this helicon wave plasma treatment apparatus is being used to physically etch a semiconductor wafer using argon, argon is first supplied from the gas supply means into the processing vessel and its pressure is adjusted to 10 mmTorr, with the semiconductor wafer held on the holder device. In this state, radio-frequency electric power is applied to the antenna, the generation of electromagnetic waves proceeding from the antenna in the axial direction within the processing vessel causes electrons to absorb energy from these electromagnetic waves, these electrons collide with the argon gas, ionizable energy is imparted to the argon atoms, and this generates the high-density plasma. At the same time, a magnetic field is shaped parallel to the direction of progress of the electromagnetic waves within the processing vessel by the electromagnetic coil. Thus, low-frequency helical waves from the electromagnetic waves are propagated through the plasma by the action of this magnetic field, and, during this time, the electrons within the plasm are accelerated to increase the density of the plasma. At the same time, radio-frequency electric power is supplied via the blocking capacitor by the radio-frequency power source of the holder device, so that the holder device becomes self-biased as described above, and the potential difference with respect to the plasma's potential causes argon ions to impact against the semiconductor wafer so that the semiconductor wafer is etched thereby. Since this plasma treatment can be implemented by causing the generation of helical waves within even a comparatively weak magnetic field, it is possible to soften the effects of the magnetic field in comparison with electronic cyclotron resonance (ECR) plasma treatment that uses a strong magnetic field.
A high-density plasma of the order of 10.sup.11 cm.sup.-3 can be obtained with the above described prior art plasma treatment apparatus that uses inductively coupled radio-frequency waves, but if an attempt is made to increase the power supplied to the antenna to further increase the density of the plasma (for example, if the supplied power is increased to 600 W), the plasma density does become approximately uniform within a central portion within a radius of 50 mm from the center of the processing vessel, but as the power is increased further to 800 W and then 1 kW to increase the plasma density even further, as will be shown later in a graph, the plasma density in the central portion drops, there is an increasing tendency for the plasma density to increase with distance from the center in a certain direction, and thus it becomes more difficult to obtain an uniform high-density plasma as the plasma density increases, raising concern that it is becoming more difficult to implement uniform plasma treatment. This tendency becomes more and more obvious as the diameters of semiconductor wafers increase, and it is becoming a big problem concerning plasma treatment as diameters become increasingly larger in the future.
If the plasma densities obtained by plasma treatment apparatuses are up to the order of 10.sup.11 cm.sup.-3, a self-bias potential of several tens of volts to several hundred volts can be obtained by just the application of power of several tens of Watts to the holder device, but when it comes to the above described higher plasma densities of the order or 10.sup.11 cm.sup.-3 or higher, conventional radio-frequency bias circuitry cannot achieve a self-bias potential of about several tens of watts, even if power is supplied at several hundred watts, so that concern has been raised that it has become impossible to implement such plasma treatment.
Further, when a prior art helical wave plasma treatment apparatus is used, plasma treatment can be implemented even with an ECR plasma in a weak magnetic field. However, if, for example, the magnetic field is further weakened in order to lessen the effects of the magnetic field, etching rates gradually decrease with distance from the antenna, etching at the central portion of the semiconductor wafer is delayed the most, and thus a problem occurs in that it becomes difficult to implement uniform etching (plasma treatment) over the entire surface. In general, trends such as the recent increase in diameter of semiconductor wafers are becoming more obvious, and film thicknesses have also become even thinner. Therefore it is becoming impossible to use a prior art helicon wave plasma treatment apparatus to treat objects under weak magnetic field conditions.
If etching of a semiconductor wafer is done using a high-density plasma with a prior art helical wave plasma treatment apparatus, there is a problem in that only the peripheral parts of the semiconductor wafer that are outside a high-density region of the plasma are etched, virtually no etching occurs in the central portion within this region, and also there is a problem that the etching rate is not constant over all positions on of the semiconductor wafer.